The present invention relates to a radio communication system and a traffic channel assignment method.
One of radio communication systems requiring the assignment of traffic channels includes an analog cordless telephone system. In the conventional analog cordless telephone system, a base station makes a successive search of a plurality of channels having predetermined frequencies prior to the communication in a mobile station so that a channel having a received signal strength level equal to or smaller than a predetermined value is judged as being an available (or empty) channel. If an available channel is found out, it is assigned as a traffic channel.
The IEICE Japan National Spring Conference, B860, 1989 has proposed a traffic channel assignment method in which a base station calculates a carrier to interference power ratio (CIR) on each channel from the received signal strength level of interference wave at that channel and the received signal strength level of transmitted wave from a mobile station to judge a channel with CIR equal to or greater than a specified or predetermined threshold value as being an available channel in order to ensure the lowest communication quality at a radio communication system. In the case where there are a plurality of available channels, a channel having the smallest CIR is selected as a traffic channel.
However, in the traffic channel assignment method as in the conventional analog cordless telephone system in which any channel having a received signal strength level equal to or smaller than the predetermined value is immediately selected as a traffic channel if it is found out, a problem is involved in the effective use of channel frequency as the whole of a radio communication system.
On the other hand, in the traffic assignment method in which available channels are selected as traffic channels in order of smaller CIR, each base station operates so that the selection as a traffic channel is made giving priority to a channel with low communication quality at another base station relatively near to that base station rather than a channel with high communication quality at another base station far from that base station. As a result, there is a merit that the same frequency channel is commonly used by a large number of base stations placed at relatively short intervals and hence the efficiency of use of channel frequency as the whole of the radio communication system is improved. In the latter traffic channel assignment method, however, since available channels are selected or assigned as traffic channels in order of smaller CIR even in the case where the traffic density is low, there is a problem that the traffic channel encounters the interference of electric waves from other base stations and hence a satisfactory radio link quality cannot be obtained.